Answers

1.- It  was snowing (snow) when I left (leave) home this morning. 
2.- It was a sunny afternoon and people were sitting (sit) on the grass in the park. Then suddenly it started (start) to rain.
 3.- A: I tried to explain my problem to her. B: Did she listen(she/listen)?
 4.- My brother was talking (talk) on the phone when I arrived, but when he saw (see) me, he finished (finish) the call. Perhaps he thought that I was listening(listen) to his conversation. 
5.- I nearly had an accident today. A car was coming (come) towards me, but I moved quickly out of the way and fortunately nothing happened (happen). 
6.- A: Which hotel did you stay (you/stay) in when you lost your passport? B: I don’t remember. I visited (visit) many places during my European tour and I stayed (stay) in many different hotels. 
7.- I moved (move) to Hastings in 1999. I was living (live) there when I met them.
 8.- How many people were waiting (many people / wait) for the 9:15 bus last night? 
9.- I was dancing (dance) with my girlfriend when the disc jockey played our favourite song. 
10.- I was studying (study) grammar when I fell (fall) asleep. 
11.- The scientists were working(work) in their laboratory when they discovered (discover) the new drug. 
12.- We were climbing (climb) the wall when the gardener saw (see) us. 
13.-Vicky was having (have) a beautiful dream when the alarm clock rang (ring)
 14.- As he was running (run) for the bus he collided (collide) with a street lamp. 
15.- When he was carrying (carry) a suitcase, he dropped (drop) it on his foot. 
16..- Were you doing (you / do) your homework on the bus while you were coming (come) to school?
 17.- The students were reading (read) the article when the last class started(finish). 
18.- When I arrived(arrive) at the cinema, my friends were waiting (wait) for me. 
19.- Were you listening (you / listen) to the teacher when she explained (explain) this exercise? 
20.- Susan was playing (play) the piano while Mary was singing (sing).